Influenza, caused by Influenza A virus of the family Orthomyxoviridae, is the most economically important disease in humans, pigs, horses, and fowls.
Influenza A viruses are further classified based on the characteristics of two surface proteins known as hemagglutinin (H) and neuraminidase (N). Influenza A virus is expressed as a combination of the H (hemagglutinin) subtype and N (neuraminidase) type one (e.g., H9N2). There are 16 different H subtypes and 9 N subtypes, resulting in a total of 144 different possible combinations of H and N subtypes of influenza A viruses.
Influenza is a zoonosis. The type A viruses are the most virulent human pathogens among the three influenza types, and cause the most severe disease. In addition, they are highly apt to mutate and can be readily transmitted from one species to another, causing pandemics. Accordingly, the breakout of pandemic influenza is emerging as a great problem to be solved. Furthermore, there are several reports that influenza viruses are infecting new species that have heretofore been known to be resistant to virus infection.
Canine influenza refers to new varieties of Influenza virus A that cause influenza in canines. Because of the lack of previous exposure to this virus, dogs have no natural immunity to this virus. Therefore, all species and ages are susceptible to this virus. Dogs with canine influenza may suffer from acute pneumonia, showing the symptoms of a severe cough, a high fever and rhinorrhea.
A highly contagious influenza virus was found to have been the cause of Greyhound race dog fatalities from a respiratory illness at a Florida racetrack in 2004. Then, as outbreaks thereof were reported in Texas, Alabama, Arkansas and other states in the U.S.A., canine influenza was regarded as a new epidemic in dogs. An epidemiological survey showed the virus, isolated from a dog with canine influenza, was almost identical to the equine influenza virus H3N8, indicating the creation of canine influenza as a result of transmission from horses to dogs. There are reports of the equine influenza virus H3N8 causing hemorrhagic pneumonia in racing dogs and of isolation of the human influenza virus H3N8 from dogs. However, sufficient serological and virological evidence must be found for canine influenza.
In addition, there have been reported cases of the outbreak of avian influenza in canines. It is inferred that the epidemiological mechanism of the transmission of influenza from birds to dogs has two routes: one is by feeding dogs with uncooked birds carrying influenza, such as ducks, chickens, etc.; and the other main way that the influenza virus is spread is from infected dogs to normal dogs in respiratory droplets of coughs and sneezes. As such, it is inferred that canine influenza is established after infected dogs are exposed to new environments and brought into contact with normal dogs. It is important to prevent canine influenza because canine influenza viruses may cause secondary infection with various mortalities. There is no vaccine available for dogs at this time.